Various types of sensors have been designed for measuring properties of gas. Some of these sensors were designed to measure a specific component of a gas in a given gas mixture. For example oxygen concentrators may be used to remove the carbon dioxide and nitrogen from air. This leaves a pre-defined mixture of oxygen and argon which is not removed by the concentrator. It is desired to indicate to the patient the purity of oxygen concentration delivered by his concentrator. It is also desirable to indicate the delivered gas flow rate which may be critical to remain constant. Sensors may be used to verify that the prescribed gas delivery requirements are met.
Douglas U.S. Pat. No. 5,060,506 describes a cylindrical chamber with piezoelectric transducers mounted on diametrically opposite sides of the chamber and are aligned along an axis inclined to the axis of gas flow through the chamber for measuring the concentration of a gas constituent. Transmitting transducer is excited by short pulse trains. Between excitations there is a waiting period needed to dispose of standing waves. However, in the Douglas apparatus a very short chamber (1.5 inches, 3.81 cm) allows only very small gas sample which increases accuracy dependence no extraneous factors such as mechanical degradation and long term drift. Low flow velocity inside the sensing chamber provides no meaningfully different travel times in and against the direction of flow. Together, these introduce low signal to noise ratio and high drift of the sensor calibration point effected by variations in mechanical properties of the chamber as a result of temperature and time.
Frola et al U.S. Pat. No. 5,247,826 describes a narrow coiled tube with two piezoelectric transducers mounted on its opposite sides and aligned along the axis of the tube. Temperature sensing thermistor wire is placed in the center of the tube. Piezoelectric transducers are energized periodically and bursts of ultrasonic energy are emitted into the tube in the direction of the receiver. The gas concentration reading is effected by the temperature of the gas, the flow rate, the path length between the sensors, and by the gas composition. The time required for the resulting ultrasonic wave to reach the other side of the tube is measured. To cancel the effect of flow on travel time, the two piezoelectric transducers alternately operate as transmitter and receiver. Travel time in the direction of flow and against the direction of flow is recorded. The two ultrasonic wave travel times and the temperature of the gas are used to calculate the standard gas flow rate and concentration of oxygen in the gas. However, this apparatus utilizes a long and flexible tube, which significantly increases the sensor size (6.times.4.5.times.1.5 inches, 15.2.times.10.1.times.3.8 cm). Tube is made of soft plastic material which can cause further complications as a result of dimensional changes in the flexible-coiled tube length as a result of temperature and time.
Aylsworth U.S. Pat. No. 5,060,514 describes a cylindrical chamber with piezoelectric transducers mounted on diametrically opposite sides of the chamber and are aliened along an axis inclined to the axis of gas flow through the chamber for measuring the concentration of a gas constituent. Transmitting transducer is energized by an endless pulse train, and phase difference between transmitted pulses and received pulses is detected at the receiver. However, this approach relies heavily on laminar flow inside the cylindrical chamber which is very difficult to achieve. Moreover, endless pulse train approach generates standing waves which introduce system inaccuracies and complicate system calibration.
Other commercially available sensors utilize galvanic or ceramic fuel sells to measure oxygen and other gas concentrations. However, these sensors require frequent calibration, their life time is limited, their response time is long, and they utilize very high temperature which entails high power consumption by the sensor (2 w to 10 w).